The beta-halo derivatives of D-alanine are very efficient inhibitors of D-amino acid transaminase. Ki values in the range of 10 - 35 microns M are found when initial velocities in the presence of substrates are measured. In the absence of substrates, the beta-haloalanine derivatives are enzyme-activated inhibitors (suicide substrates). Cycloserine is a very efficient active site titrant for the transaminase with incorporation of 1 mole of cycloserine per molecule of dimeric enzyme. Gabaculine is also an inactivator of the transminase and possesses some antibacterial properties as well. The primary structure of D-amino acid transaminase is being determined.